The oviductal epithelium in the human and other mammals undergoes a cyclic differentiation process during each menstrual or estrous cycle. Although estrogen is known to induce differentiation, and progesterone dedifferentiation, little is known regarding the temporal sequence of molecular events controlling these responses. A thorough analysis of the regulatory mechanism(s) involved has been hindered by the fact that the most commonly used and easily maintained laboratory animals exhibit a short estrous cycle where the differentiation-regression cycle is not complete nor well delineated from one cycle to the next. In contrast, however, the oviductal epithelium in the female beagle bitch undergoes a complete and distinct morphogenetic cycle during each estrous cycle that is characterized by: 1) the transition of undifferentiated, low cuboidal cells into columnar ciliated and secretory cells; and, 2) the subsequent dedifferentiation and return of these cells to the undifferentiated state. In addition, the exact time course and sequence of these events can be precisely mimicked by the proper administration of exogenous estrogen and progesterone to prepubertal female pups. The prepubertal pup oviduct, therefore, is an excellent model for studying the timing and basic subcellular mechanisms controlling differentiation and regression of the oviductal epithelium in mammals. We intend to use this model to determine the role of estrogen and progesterone receptors in the cyclic phenomena of differentiation, growth and regression in the oviduct. A comprehensive and systematic analysis of estrogen and progesterone receptor characteristics (i.e., ontogeny, quantity, and subcellular compartmentalization) during estrogen stimulated differentiation, and progesterone induced regression, will be conducted. We anticipate that the information obtained from the present study will provide a more complete understanding of how the steroid hormones, estrogen and progesterone, acting in concert with their respective receptors are involved in (and possibly control) the development and functional state of the mammalian oviductal epithelium.